Microfluidic Systems For Electrochemical Transdermal Analyte Sensing Using a Capillary-Located Electrode

ABSTRACT

A sensing device, designed to be used in contact with the skin, contains a plurality of individually controllable sites for electrochemically monitoring an analyte, such as glucose, in interstitial fluid of a user. The device includes at least a hydrophobic layer designed to contact the skin; a capillary channel providing an opening adjacent the skin; a metal electrode layer having a sensor layer applied to an edge portion thereof such that it is exposed to the interior of said capillary channel, the sensing layer being effective to measure the analyte.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 13/834,199filed Mar. 15, 2013 and titled “Microfluidic Systems For ElectrochemicalTransdermal Analyte Sensing Using a Capillary-Located Electrode,” whichis incorporated herein by reference in its entirety.

FIELD OF EMBODIMENTS

The present embodiments relate generally to non-invasive or minimallyinvasive transdermal measurement systems. More specifically, theembodiments relate to microfluidic transdermal glucose measurementsystems in which a thin electrode is contained within afluid-transmitted capillary, and processes for their production and use.

BACKGROUND

Minimally invasive transdermal systems are described in, for example,co-owned U.S. Pat. Nos. 6,887,202 and 7,931,592, both entitled “Systemsand Methods for Monitoring Health and Delivering Drugs Transdermally,”as well as co-owned U.S. application Ser. No. 13/459,392, each of whichis incorporated herein by reference in its entirety. These systems, likethe embodiments described herein, provide for a minimally invasivesampling technique and device suitable for rapid, inexpensive,unobtrusive, and pain-free monitoring of important biomedical markers,such as glucose.

Existing systems remain open to improvement in various aspects,including consistency in sampling and measurement.

SUMMARY

A sensing device, designed to be used in contact with the skin, isprovided. The device contains a plurality of individually controllablesites for electrochemically monitoring an analyte, such as glucose, ininterstitial fluid of a user. The device includes:

a hydrophobic layer, designed to contact the skin; an overlaying firststructural layer;

an overlaying metal electrode layer;

an overlaying second structural layer;

for each such detection site, a capillary channel traversing theselayers, thus providing an opening adjacent the skin;

wherein said metal electrode layer is discontinuous at the circumferenceof said capillary channel, such that two non-contiguous edge portions ofelectrode are present within the circumference of said channel;

applied to one such edge portion of the metal electrode layer, such thatit is exposed to the interior of said capillary channel, a sensing layereffective to measure said analyte; and

surrounding the lower end of said capillary channel, adjacent saidhydrophobic layer, an electronic element (microheater) effective toproduce heat when a sufficient voltage is applied thereto.

Also provided are electrical conduits and contacts such that a voltagecan be applied to the microheater, and an additional voltage can beapplied between the two edge portions of the electrode layer, and anelectrochemical response from the sensing material/electrode layer,indicative of the concentration of analyte in the sample fluid, can bedetected.

In selected embodiments, the hydrophobic layer is hydrophobic silicone.The first structural layer may be a glass or ceramic-like material. Themetal electrode layer is preferably gold or platinum, and the sensinglayer, for use in detecting glucose, is preferably a conducting polymer,such as polypyrrole (PPy), modified with glucose oxidase (GOx), andpreferably further containing an effective amount of a mediator such asferricyanide. The second structural layer is preferably non-absorbentand/or hydrophobic, and may also be a layer of hydrophobic silicone.

The diameter of the capillary channel, in one embodiment, is about 50μm.

The thickness of the metal electrode layer is generally in the range of100 nm to 1 micron range, e.g. 100-500 nm, 500-1000 nm, 500-800 nm,250-750 nm, 300-500 nm, etc. An exemplary thickness is about 500 nm. Thestructural layers generally have thicknesses such that the overallthickness of the device is about 1 mm or less.

The thickness of the applied sensing layer, measured in a directionperpendicular to the capillary channel length, may be 200 nm or less,100 nm or less, or 50 nm or less, in selected embodiments.

In use, a voltage is applied to the microheater sufficient to ablate thestratum corneum of the underlying skin, e.g. a voltage of about 3 V,typically for about 30 msec. This ablation allows interstitial fluid toenter the capillary channel, where it rises via both capillary actionand the body's hydrostatic pressure and contacts the sensing material(e.g. PPy/GOx) within the capillary. A second voltage, typically 0.2-0.4V, is applied to the electrode layer, i.e. between the twoabove-described edge portions of the electrode layer, and the level ofanalyte (e.g. glucose) contacting the sensing material iselectrochemically detected, in accordance with known methods.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 illustrates an embodiment of a sensing device as disclosedherein.

DETAILED DESCRIPTION

A section of an exemplary sensing device, designed to be used in contactwith the skin, is shown in FIG. 1. The device typically contains aplurality of individually controllable sites, of which one isillustrated in the FIGURE, for electrochemically monitoring an analyte,such as glucose, in interstitial fluid of a user. The device, in apreferred embodiment, includes:

a hydrophobic layer 12, designed to contact the skin;

an overlaying first structural layer 14;

an overlaying metal electrode layer 16;

an overlaying second structural layer 18;

a capillary channel 20 traversing these layers, thus providing anopening adjacent the skin;

wherein said metal electrode layer is discontinuous at the circumferenceof said capillary channel, such that two non-contiguous edge portions ofelectrode are present within the circumference of said channel;

applied to one such edge portion of the metal electrode layer, such thatit is exposed to the interior of said capillary channel, a sensing layer22 effective to measure said analyte; and

surrounding the lower end of said capillary channel, adjacent to saidhydrophobic layer, an electronic element (microheater) 24, effective toproduce heat when a sufficient voltage is applied thereto.

The diameter of the capillary channel, in one embodiment, is about 50μm.

The thickness of the metal electrode layer is generally in the range of100 nm to 1 micron range, e.g. 100-500 nm, 500-1000 nm, 500-800 nm,250-750 nm, 300-500 nm, etc. An exemplary thickness is about 500 nm. Thethickness of the structural layers is not generally critical (althoughlayer 12 should be sufficiently thick to insulate sensing material 22from heat produced by microheater 24), but these may also be in thegeneral range of hundreds of nm, e.g. 100-500 nm, 500-1000 nm, 500-800nm, 250-750 nm, 300-500 nm, etc. The overall thickness of the device isgenerally less than 1 mm.

The diameter of the capillary channel 20, in one embodiment, is about 50μm. Other diameter ranges, e.g. 10-100 μm, or 25-75 μm, could also beeffective.

Also provided, though not shown in the FIGURE, are electrical conduitsand contacts such that a voltage can be applied to the microheater, andan additional voltage can be applied to the electrode layer (i.e.between the two above-described edge portions of the electrode layer),and an electrochemical response from the sensing material/electrodelayer, indicative of the concentration of analyte in the sample fluid,can be detected. The multiple detection sites within a device arepreferably individually controllable; i.e. voltages can be selectivelyapplied to a given detection site or sites by a user of the device.

In selected embodiments, the hydrophobic layer 12 is hydrophobicsilicone, though any biocompatible/non-irritating hydrophobic materialcan be used. The structural layer 14 may be a glass or ceramic-likematerial, which provides thermal insulation between the microheater 24and sensing material 22, or other structurally stable, nonabsorbent,preferably thermally insulating material. The metal electrode layer 16is preferably gold or platinum.

A sensing layer 22 effective to measure the analyte is present on one ofthe above-described edge portions of the metal electrode layer, suchthat it the sensing material is exposed to the interior of the capillarychannel. The sensing layer 22, for use in detecting glucose, ispreferably a conducting polymer, such as polypyrrole (PPy), modifiedwith the enzyme glucose oxidase (GOx).

Preferably, in fabrication, the PPy-GOx layer is electrodeposited, inaccordance with known methods (see, e.g., Liu et al., Matl. Sci. Eng. C27(1):47-60 (January 2007); Yamada, et al., Chem. Lett. 26(3):201-202(1997); Fortier, et al., Biosens. Bioelectronics 5:473-490 (1990)) as anextremely thin layer on an exposed face of the metal electrode, as shownin the FIGURE. Measuring in the direction perpendicular to the capillarylength, the thickness of the applied layer may be, e.g. 200 nm or less,100 nm or less, or 50 nm or less, in selected embodiments.

In one embodiment, a mediator such as ferricyanide, as known in the art,is co-deposited along with the PPy and GOx. This system allowselectrochemical measurement of the analyte to be carried out at avoltage of about 0.2-0.4V. A somewhat higher voltage (e.g. 0.7 V), whichcan lead to interference from other molecules in the interstitial fluid,would typically be required without the mediator. Otherelectron-accepting mediators known in the art for use with GOx includeferrocene derivatives, conducting organic salts such astetrathiafulvalen-tetracycloquinodimethane (TTF-TCNQ), quinonecompounds, phenothiazine compounds, and phenoxazine compounds.

The multilayer structure of the device containing the capillary channelscan be fabricated by known deposition and etching methods. The sensingmaterial 22 is, in one embodiment, applied by electrodeposition to oneof the exposed edges of the gold electrode within the formed capillarychannel 20, as noted above. As noted above, two noncontiguous edges ofelectrode are present within the microchannel, to allow forelectrochemical detection. These edges could be visualized as twodistinct semicircles within the inner surface of the channel, one ofwhich is treated with the sensing material.

In use, a voltage is applied to the microheater sufficient to ablate thestratum corneum of the underlying skin, e.g. a voltage of about 3 V,typically for about 30 msec. This ablation (which typically produces atemperature of about 130° C.) allows interstitial fluid to enter thecapillary channel, where it rises via capillary action and hydrostaticpressure and contacts the sensing material (e.g. PPy/GOx) within thecapillary. A second voltage, typically 0.2-0.4 V, is then appliedbetween the two above-described edge portions of the electrode layer),and the level of analyte (e.g. glucose) contacting the sensing materialis electronically detected, preferably amperometrically detected, inaccordance with known methods.

The device design presents various advantages, including the following.The sensor electrode pair, including the metal (e.g. gold) electrode andPPy/GOx-treated electrode (i.e. the two edge regions described above,one treated with sensor material), are located within the microcapillarychannel and thus separated spatially from the microheater. Thisconfiguration avoids possible heat degradation of the enzyme. Further tothis aspect, structural layer 14 is preferably formed of aheat-insulting material, such as a glass or ceramic material.

The detection of glucose is typically realized using chronoamperometry(measurement of current generated versus time for a voltage step).Ideally, every glucose molecule reaching the GOx sensing electrodeshould immediately release its electrons to produce the measuredcurrent. To achieve this condition, the electrode should have a lowsurface area to sample volume ratio, to ensure that glucose is notdepleted in the vicinity of the sensing electrode during analysis.Accordingly, the sensing electrode is fabricated to be extremely small;i.e. essentially the width of the metal electrode layer 16, as shown inthe FIGURE. Preferred thicknesses (measured in the directionperpendicular to the channel length) of the applied sensor layer 22 maybe, e.g. 200 nm or less, 100 nm or less, or 50 nm or less, in selectedembodiments, in the 100 nm to 1 micron range, e.g. 500 nm. Diffusiontimes (i.e. the time for glucose molecules to reach the GOx enzyme) arereduced for similar reasons.

In general, the thickness of a metal layer applied via conventionalmetal deposition methods, e.g. electrodeposition or vapor deposition,can be precisely controlled, as compared to control of lateraldimensions of the planar surface area. Accordingly, high consistency inthe effective sensor area (which is, again, the width dimension of themetal electrode layer 16), as well as roughness of the electrode layer,is achieved, giving high consistency between one sensor element andanother, within a single device or between different devices. Infabrication of the multilayer device, the gold thickness can be easilyreproduced with very little sidewall imperfections/roughness, and theexposed region (at the capillary wall) becomes the sensor electrodearea.

Although glass/ceramic/polymeric substrate layers are exemplified, othermaterials, such as paper or other cellulose substrates, electrospunfibers, or other polymers, could also be used for the non-metal layers(12, 14, 18) in the device. However, surfaces contacting the skin, suchas the lower surface of layer 12, should be non-absorbent and preferablyhydrophobic in nature, in order to direct fluid flow from the skin intoand though the capillary channel 20 to the sensing material 22. Methodsof treating materials such as paper to render selected portionshydrophobic and/or non-absorbent are known in the art; see, e.g.Martinez, et al., Anal. Chem. 2010, 82, 3-10. The surface of structurallayer 12 contacting the interior of the microchannel should benon-absorbent but should not repel water, so that sample fluid travelsefficiently to the sensing area without volume loss.

Integrated circuitry (IC), including radio frequency (RF) communicationcapability, may be included peripheral to the device in order totransmit data readings to a remote location. By way of example, thistransmission may employ Bluetooth devices, or it may be facilitated aspart of a home area network (HAN) in a first instance, e.g., usingprotocols such as those described as part of the Zigbee standards.Further still, the data readings may be further transmitted outside ofthe HAN in accordance with a home health or telehealth communicationssystem using existing wide area networks (WANs) such as the Internet.

One skilled in the art recognizes the other areas of application for thedevices described herein. While the examples specifically describedherein are directed to glucose monitoring, adaptations could be made toascertain other information from the biomolecules and biomarkers in theinterstitial fluid. For example, the individual sites could monitor forinfectious disease (microbial, fungal, viral); hazardous compounds;heart or stroke indicators (troponin, C-reactive protein); chemical orbiological toxins; cancer markers (PSA, estrogen); drug efficacy anddosing (metabolites): and the like. Such applications of the device asdescribed are considered to be within the scope of the presentinvention.

1. A sensing device comprising a plurality of individually controllabledetection sites for electrochemically monitoring an analyte ininterstitial fluid of a user, each individually controllable detectionsite comprising: a dual opening capillary channel traversing multiplelayers and having one of the dual openings located adjacent to the skinof the user, the multiple layers including at least a hydrophobic layer,designed to contact the skin, and a metal electrode layer, wherein themetal electrode layer is discontinuous at a circumference of thecapillary channel, such that two non-contiguous edge portions of themetal electrode layer are present within the circumference of saidchannel; and a first non-contiguous edge portion of the metal electrodelayer including a sensing layer applied thereto and being exposed to aninterior of the capillary channel, wherein the sensing layer iseffective to measure the analyte within interstitial fluid of the userentering the capillary channel and contacting the first non-contiguousedge portion including the sensing layer.
 2. The sensing device of claim1, further comprising: a microheater located adjacent said hydrophobiclayer, the microheater being effective to produce heat when a sufficientvoltage is applied thereto to ablate the stratum corneum of theunderlying skin and access the interstitial fluid containing theanalyte.
 3. The sensing device of claim 1, wherein the hydrophobic layeris silicone.
 4. The sensing device of claim 1, wherein the firststructural layer is selected from the group consisting of glass and aceramic-like material.
 5. The sensing device of claim 1, wherein themetal electrode layer is selected from the group consisting of gold andplatinum.
 6. The sensing device of claim 5, wherein the sensing layer isa conducting polymer.
 7. The sensing device of claim 6, wherein theconducting polymer is polypyrrole (PPy).
 8. The sensing device of claim7, wherein the polypyrrole (PPy) is modified with glucose oxidase (GOx).9. The sensing device of claim 8, wherein the polypyrrole (PPy) ismodified with glucose oxidase (GOx) is co-deposited on the edge of themetal electrode layer with a mediator.
 10. The sensing device of claim9, wherein the mediator is ferricyanide.
 11. A method forelectrochemically monitoring an analyte in interstitial fluid of a user,the method comprising: contacting the user's skin with a monitoringdevice, the monitoring device including a plurality of individuallycontrollable detection sites for electrochemically monitoring theanalyte in interstitial fluid of a user; controlling at least a first ofthe individually controlled detection sites to apply a first voltage toa microheater located adjacent to the user's skin at the first of theindividually controlled detection sites, wherein the microheaterproduces heat responsive to the applied first voltage, the heat beingsufficient to ablate the stratum corneum of the underlying skin andaccess the interstitial fluid of the user containing the analyte;receiving the accessed interstitial fluid at a first opening of a dualopening capillary channel of the first of the individually controlleddetection sites, wherein the accessed interstitial fluid rises throughthe capillary channel, traversing multiple material layers including atleast a hydrophobic layer contacting the user's skin, first structurallayer and a metal electrode layer that is discontinuous at acircumference of the capillary channel, such that two non-contiguousedge portions of the metal electrode layer are present within thecircumference of said capillary channel, a first non-contiguous edgeportion of the metal electrode layer including a sensing layer appliedthereto and being exposed to an interior of the capillary channel;applying a second voltage between the two non-contiguous edge portionsof the metal electrode layer when the interstitial fluid passes therebywithin the capillary channel; and electronically detecting the analytein the interstitial fluid using the sensing layer responsive to theapplication of the second voltage.
 12. The method of claim 11, furthercomprising applying the first voltage for approximately 30 msec.
 13. Themethod of claim 12, wherein the first voltage is about 3V.
 14. Themethod of claim 11, wherein the second voltage is about 0.2 to 0.4 V.15. The method of claim 11, wherein the hydrophobic layer is silicone.16. The method of claim 11, wherein the first structural layer isselected from the group consisting of glass and a ceramic-like material.17. The method of claim 11, wherein the metal electrode layer isselected from the group consisting of gold and platinum.
 18. The methodof claim 17, wherein the sensing layer is a conducting polymer.
 19. Themethod of claim 18, wherein the conducting polymer is polypyrrole (PPy).20. The method of claim 19, wherein the polypyrrole (PPy) is modifiedwith glucose oxidase (GOx).